1. Field of the Invention
The present invention relates to the improvement of a molded eccentric rotor employed, for example, as silent call means in portable communication devices and to the configuration of a flat and thin coreless vibration motor of an axial-gap type which comprises the rotor.
2. Description of the Prior Art
A molded eccentric rotor in which vibrations are generated by centrifugal force of the rotor itself is composed of a flat crescent-shaped eccentric rotor in which an air-cored armature coil is shifted to one side. Such an eccentric rotor has been suggested long time ago by the applicant of the present invention in JP Published Application No. 6-205565.
However, miniaturization and width reduction of portable communication devices have also created a demand for smaller and thinner components installed in portable communication devices. For example, if a diameter of 12 mm and a thickness of about 3 mm are required, then in a flat crescent-shaped eccentric rotor in which an air-cored armature coil is shifted to one side and which conventionally had an advantageous structure with a diameter of about 14 mm, the displacement of the center of gravity is small and the centrifugal vibrations are insufficient, making it necessary to employ an auxiliary weight composed of a component with a high specific gravity.
For this purpose, the applicant has suggested a structure in which the required weight and displacement of the center of gravity is obtained by the so-called two-color molding in which a component with a high specific gravity which is composed of a tungsten alloy powder or block is molded with a resin (JP Published Application No. 11-73542).
In such a molded eccentric rotor, a guide hole matching the diameter of the corresponding air-gap armature coil and a first-turn connection pattern are provided in a printed wiring board where the air-gap armature coil is to be installed, and after the air-gap armature coil has been installed, the printed wiring board is turned downward and integrated molding is conducted by injection molding from the side of the upper air-gap armature coil (JP Published Application No. 8-16346 and No. 11-113231).
For cost reduction, it was suggested to remove one of the three three-phase air-gap armature coils, to shift the remaining two flat coils to one side, and further increase the eccentricity by auxiliary disposing a member with a high specific gravity at the displacement side (JP Published Application No. 2000-286030).
However, though such a structure was effective in motors with a diameter of about 14 mm, when it was employed in motors with a diameter of about 10 mm and a thickness of about 2 mm in configurations with an auxiliary weight, especially such that it protruded to a magnet, the rotor thickness was sacrificed and the rotor was difficult to employ. Another drawback was that in configurations with an arrangement opening angle of the two coils of 120 degrees, the portion of the auxiliary weight was small and the eccentricity as a whole was small.
Further, it order to use a flat vibration motor equipped with such an eccentric rotor as silent call means of a portable communication device, a mounting surface has to be flat to provide for direct installation via a two-sided adhesive or the like on the printed wiring board of the device.
Moreover, a power supply terminal for supplying electric power to a brush is led out to the side peripheral portion of the motor and connected to the power supply pattern provided in the printed wiring board of the device.
Such flat motors are driven with an axial field-type ring-shaped magnet. Therefore, proper attention has to be paid to the power supply structure for supplying power to a brush disposed in the inner portion of the magnet.